1. Field of the Invention
The invention relates to a line terminating device as claimed in the preamble of patent claim 1.
2. Description of the Related Art
Voice is transmitted in analog form via an analog subscriber access in a telephone network. This is done using a frequency band, referred to as the voice band, which represents only a relatively narrow frequency band from the entire transmission bandwidth of a double copper wire (POTS=Plain Old Telephone System). In what are referred to as the xDSL transmission methods (xDSL=x-Digital Subscriber Line), the frequency bands above the voice band are also used for broadband data transmission in a telephone network.
xDSL transmission methods include HDSL (High Bit Rate Digital Subscriber Line), ADSL (Asymmetric Digital Subscriber Line) and VDSL (Very High Speed Digital Subscriber Line). xDSL transmission methods are referred to as broadband network access (broadband access technology) and include all transmission methods via a telephone network which allow a higher data transmission rate between a subscriber and the central office (CO) than the data transmission rate achievable using voice band modems. In xDSL transmission methods, what is referred to as xDSL data signal, which denotes a digital signal coded for the xDSL transmission method, is transmitted for this purpose in a higher frequency band, the data band, which is separated from the voice band. In theory, xDSL transmission methods can use the entire bandwidth available above the voice band with double copper wires, and can achieve data transmission rates in the Mbps range.
Despite voice and data being transmitted separately in different frequency bands, interference frequency signals from the voice band can enter the data band, and vice versa, in xDSL transmission methods. Control signals, which are used in particular in the voice band, such as ringing signals and charge pulses, can cause interference in the data band and can interrupt, or even entirely terminate, data transmission. To avoid such interference, the voice and data bands must be separated before further processing. Frequency splitters, which are referred to as POTS splitters, are required for this purpose which, at the receiving end, separate a broadband signal into a narrowband low-frequency voice signal and a broadband higher-frequency data signal and, at the transmission end, filter voice signals and data signals, and join them together to form a broadband signal.
A POTS splitter is known from U.S. Pat. No. 5,757,803, which has an analog low-pass filter and two capacitances, with the low-pass filter, which has a passband for the voice band, being connected upstream of an analog subscriber terminal, and capacitances which suppress the two control signals from the voice band being connected, upstream of an xDSL transmitter/receiver. The xDSL transmitter/receiver, which is connected via the two capacitances to a subscriber access line, contains further analog high-pass filters which, together with the two capacitances, separate the data band from the voice band.
An ADSL transmitter/receiver is known from U.S. Pat. No. 5,742,527, which receives and transmits a broadband signal via a subscriber access line and, in the process, filters an analog voice signal out of the received signal using a bandpass filter. An ADSL signal which is to be transmitted is likewise filtered using a bandpass filter, in order to remove from the voice band interference frequencies which can pass from the ADSL signal to the voice band, and in order not to influence the voice band transmission. Following an analog/digital converter, the ADSL receiver contains a decimator, which filters an ISDN signal which is present in the signal out of the received signal. The ADSL transmitter has an interpolator with a high-pass filter function, which is arranged upstream of a digital/analog converter and filters interference frequency signals which are in the voice band out of an ADSL signal which is to be transmitted.
These known solutions have the disadvantage that analog filters are always required, which are formed from active and/or passive elements. Since steep filter flanks are necessary in order to separate the voice band and data band, higher-order filters must be used, which are highly complex and expensive when in the form of analog filters. Furthermore, a solution using analog filters can be matched only with great difficulty to specification changes, for example when the separation frequency between the voice band and data band should be changed.
A line access device which is advantageous in comparison to this has been proposed in German Patent Application DE 198 44 941.0, in which the use of analog separating filters is avoided and, in consequence, matching to specification changes can also be carried out more easily.
This line access device contains an analog/digital converter, which converts an analog broadband received signal, supplied to it, to a digital received signal, a digital/analog converter which converts a digital transmission signal to an analog broadband transmission signal, and a digital frequency splitter, which follows the analog/digital converter and the digital/analog converter and which separates the digital received signal into a first digital voice signal and a first digital data signal, and joins a second digital voice signal and a second digital data signal together to form the digital transmission signal.
In consequence, the broadband received signal is in this case separated into a first digital voice signal and a first digital data signal, and the second digital voice signal and the second digital data signal are joined together to form the broadband transmission signal using digital means. However, despite all the advantages, this proposed solution still has mutual interactions between the two transmission systems, namely that for transmitting data signals and that for transmitting voice signals.